Electron density in the quiet solar coronal transition region from SoHO/SUMER measurements of S VI line radiance and opacity

Abstract

Context. The steep temperature and density gradients that are measured in the coronal transition region challenge the model interpretation of observations. Aims. We derive the average electron density n e in the region emitting the S vi lines. We use two different techniques, which allow us to derive linearly-weighted (opacity method) and quadratically-weighted (emission measure method) electron density along the line-of-sight, to estimate a filling factor or derive the layer thickness at the formation temperature of the lines.Methods. We analyze SoHO/SUMER spectroscopic observations of the S vi lines, using the center-to-limb variations in radiance, the center-to-limb ratios of radiance and line width, and the radiance ratio of the 93.3−94.4 nm doublet to derive the opacity. We also use the emission measure derived from radiance at disk center.Results. We derive an opacity τ 0 at S vi 93.3 nm line center of the order of 0.05. The resulting average electron density n e , under simple assumptions concerning the emitting layer, is 2.4 × 10 16 m −3 at T = 2 × 10 5 K. This value is higher than (and inconsistent with) the values obtained from radiance measurements (2 × 10 15 m −3 ). The last value corresponds to an electron pressure of 10 −2 Pa. Conversely, taking a classical value for the density leads to a too high value of the thickness of the emitting layer. Conclusions. The pressure derived from the emission measure method compares well with previous determinations. It implies a low opacity of between 5 × 10 −3 and 10 −2 . It remains unexplained why a direct derivation leads to a much higher opacity, despite tentative modeling of observational biases. Further measurements in S vi and other lines emitted at a similar temperature should be completed, and more realistic models of the transition region need to be used.

Density diagnostics derived from the O iv and S iv intercombination lines observed by IRIS

“…This is not the case for the O iv and S iv intercombination lines, due to the low A-values of these transitions. The optical depth can in fact be easily estimated by using the classical formula given for instance in Buchlin & Vial (2009). We found that at a density of ≈ 10 13 cm −3 , the O iv lines reach an opacity of 1 over an emitting layer of the order of 10 5 km, which is much higher than the source size (≈ size of the IRIS pixel, i.e.…”

Effects of resonant scattering of the Si IV doublet near 140 nm in a solar active region

“…Solving Eq. (9) numerically (Dere & Mason 1993;Buchlin & Vial 2009), one gets a relation between τ 13 0 and the total intensity line ratio. The computed τ 13 0 values are in the range of 1.6-4.7.…”

“…Another difficulty in the diagnostic of the CCTR plasma arises because the plasma is often affected by non-negligible optical thickness. The measurement of spectral line opacities in the transition region has been pursued with different techniques (Dumont et al 1983;Fischbacher et al 2002;Buchlin & Vial 2009). Recently, with the use of the Interface Region Imaging Spectrograph (IRIS mission;De Pontieu et al 2014), more studies of opacity effects in the CCTR have been published (Yan et al 2015).…”

The Contribution of The Joule-Thomson Effect To Solar Coronal Heatin

“…The estimated electron density for the TR as a whole is 10 15 m -3 [21] for problems created by sharp density and temperature gradients) compared with 10 19 m -3 [19] for the chromosphere, a cumulative fall rather more substantial than 1/40 but consistent with the progressive change in electron density (Fig. 2) represented by typical values of 10 23 m -3 for the photosphere, 10 15 m -3 for the TR as a whole, as we saw, 10 14 for the base of the corona base in quiet regions, 10 12 m -3 at 1 R ☉ , 10 7 m -3 at 1 AU and 10 6 m -3 in the interstellar medium [19,22].…”